There are many conventional applications requiring the measurement of fluid (e.g., liquid or gas) parameters, such as fluid level, pressure, temperature, density, etc., within containers. One exemplary application is storage tanks (both above ground and underground) used to store fuel. For example, most gasoline stations have one or more underground storage tanks below ground to store the gasoline available for sale to customers. These tanks may range in size (e.g., 20,000 gallons) and in use, generally contain a stratified fuel sitting atop an inch or two of water.
Due to the flammable nature of fuel and its potential harmful impact on the environment, governmental regulations may require, and the owners may desire, the monitoring of certain parameters (e.g., fluid level) of the fuel contained within the tank to detect any leakage of fuel from the tank to enable the appropriate actions to be taken to prevent any further leakage. For example, contemporary EPA standards state that a change in fuel level greater than 0.1 gallons/hour constitutes a leak. There are a variety of sensing devices and systems designed to, for example, measure the fuel level, temperature and/or density within these tanks, which may then be used for fluid volume and tank leak detection calculations. There are also sensing devices and systems designed to monitor various parameters around the tanks. As is customary, the sensing devices may be operatively coupled to a central controller that receives signals from the sensing devices indicative of the state of the fuel within the tank and/or parameters surrounding the tank.
Because of the volatile and hazardous nature of the materials, power to the sensing devices may be limited by intrinsic safety barriers. Intrinsic safety barriers are generally devices that limit current, voltage, and total energy delivered to sensing device located in the hazardous area. Limiting the power to the sensing devices helps to prevent or reduce the likelihood of fires and explosions from occurring in areas containing hazardous chemicals, gases, or other ignitable atmospheres. While being necessary or desirable, these barriers add cost and complexity to the installation and set up of sensing device networks used in monitoring the storage tanks and surrounding area. Conventional networks contain sensing devices (e.g., probes, sensors, etc.) that are individually connected to their own dedicated barrier and then to a controller. In addition to the cost of the barrier for each sensing device, there is the additional cost and complexity of the cabling between the sensing device, barrier, and controller. Moreover, the voluminous amount of cabling required for conventional sensing device networks, and the complexity of its installation, requires considerable labor and expense. Furthermore, controllers must also be configured to have a separate, dedicated port for each of the sensing devices in the network, which may further increase the cost, complexity and labor associated with such sensing device networks.
In addition to that provided above, conventional sensing device networks require a significant amount of operator interaction. By way of example, when installing such a network, an operator will typically have to manually input various data about each of the sensing devices into the controller. This data may include voltages, currents, and various threshold values so that the controller knows how to use the data coming from the sensing device to determine if the sensing device is in alarm. A wrong threshold value may render the sensing device inoperable for its intended purpose. Moreover, such operator errors may be difficult to detect.
Additionally the installer is required to manually compare the ISB voltage and current parameters to the device's voltage and current parameters, and manually calculate the total capacitance and inductance of all the devices and again compare the total with the maximum safe value for the ISB. An error in the calculation of the inductance and capacitance may unintentionally result in potentially putting the sensing device in an unsafe condition, which could remain undetected.
Accordingly, one objective of the invention is to provide an improved sensing device arrangement that reduces the complexity of connecting sensing devices to a controller, not only in the number of network cable runs, but also in the number of wires within a cable.
A further objective of the invention is to reduce the amount of operator configuration at the controller after installing a sensing device.
Another objective of the invention is to ensure that the site is safe to operate by allowing the controller to check the entity parameters of the intrinsic safety barriers and compare them to the connected sensing devices to ensure that the parameters are not exceeded.